The invention relates to a thermoelectric element with at least one n-layer and at least one p-layer of one or more doped semiconductors, wherein the n-layer(s) and the p-layer(s) are disposed so that at least one pn-junction is formed. At least one n-layer and at least one p-layer are electrically selectively contacted, and a temperature gradient is established or obtained parallel (x-direction) to the boundary layer between at least one xe2x88x92 and p-layer.
The thermoelectric effect has already been known for more than 100 years. There is a broad spectrum of materials which can be used for the direct conversion of a temperature gradient into electric current. The technical conversion of this effect has hitherto always been based on a common basic structure (FIG. 1). Two different metals (a, b) or two differently (n- and p-) doped semiconductors are connected at one end, normally the hot end (temperature T1). At the other, normally cold end (temperature T2), the current can then be measured (resistance R as symbolic load). Such thermoelectric elements are known from, for example, the specifications EP 0 969 526 A1, JP 11195817 A, JP 10144969 A, JP 10022531 A, JP 10022530 A, JP 57-1276 (A), JP 07038158 A, JP 59-980 (A), JP 57-169283 (A), JP 4-63481 (A) and U.S. Pat. No. 5,009,717. In some cases, a conductive layer is incorporated as a contact surface between the xe2x88x92 and p-layer in the region of the p-n junction. A common feature of all these thermoelectric elements is that the p-n junction is formed only in a small region between the xe2x88x92 and p-layers whereas the larger region between the xe2x88x92 and p-layer is formed as an air gap or as an isolating layer (JP-63481 (A) and U.S. Pat. No. 5,009,717).
To obtain the most effective possible conversion of the temperature gradient into electric current, the thermoelectric elements are assembled to form a module, in such a manner that the individual elements are electrically switched in series, but thermally parallel. These modules can in turn be combined to form larger units (FIG. 2).
The choice of materials used is made according to the point of view of the maximum possible degree of efficiency in the targeted temperature range. The degree of efficiency is generally characterized by the figure of merit Z=S2/xcfx81k (S . . . Seebeck coefficient or absolute differential thermoelectric force, xcfx81 . . . specific resistance, k . . . heat conductivity). A high degree of efficiency is achieved in a material with a high Seebeck coefficient with simultaneous low specific resistance and low heat conductivity.
The prior art has some disadvantages. The properties of a material which are important for thermoelectricity (S . . . Seebeck coefficient, xcfx81 . . . specific resistance, k . . . heat conductivity) can only be influenced independently of one another to a very small extent. This connection limits the degrees of efficiency currently achievable to approximately 10-20%. In addition, the development of the temperature gradient in the state of the art has scarcely any influence on the degree of efficiency because, in conventional thermoelectric elements, only the total difference in the temperature between the hot and cold side plays a role, due to the linear connection between thermoelectric force and temperature difference. Furthermore, attempts to use alternative concepts such as, for example, diodes (pn-junctions), so that one side of the pn-junction is warmer than the other side, do show an increase in the degree of efficiency, but the original concept has not been essentially extended.
The object of the invention is therefore to create an improved thermoelectric element. According to the invention, this object is achieved in that at least one pn-junction is essentially formed along the total, preferably longest extent of the n-layer(s) and p-layer(s), and thus essentially along the whole boundary layer. A central, fundamentally novel idea is to use at least one pn-junction, with the temperature gradient developing along the pn-junction which has a corresponding longitudinal extent.
In the state of the art, the pn-junction is formed only in a small contact region with a constant temperature, mostly on the high temperature side of the thermoelectric element. It thus serves only to improve the electric contact between the xe2x88x92 and the p-doped parts (layers). In contrast to this, according to the present invention, at least one pn-junction is essentially formed over the whole extent of the xe2x88x92 and p-layers, with a temperature gradient being established along the pn-junction boundary layer. This gives rise to a temperature difference along this elongated pn-junction between two ends of a player package, which leads to the degree of efficiency of the thermoelectric element according to the present invention being distinctly higher than in the state of the art, which has no temperature gradient along and within the p-n junction. The more precise method of operation is based on the different type of formation of potential modulations in a pn-junction at different temperatures, as explained below with reference to the description of the figures.
What is important for the function principle of this novel type of thermoelectric element is the selective contacting of xe2x88x92 and p-layer. This can preferably be carried out either by alloying of the contacts and the pn-junctions connected to them, or by direct contact of the individual layers.